It is generally appreciated that Fourier Transform Infra Red (FTIR) Spectroscopy techniques provide excellent tools for measuring the concentrations of a large number (e.g. 20+) gasses simultaneously, in real-time. The term “real-time” as used herein typically refers to reporting, depicting, or reacting to events at the same rate and sometimes at the same time as they unfold, rather than delaying a report or action. Because of this FTIR has been an important technology for emissions monitoring industries, however there have been challenges to reliably use FTIR for certain portable or industrial applications in environments that produce forces that interfere with FTIR performance. One example of which includes a system to measure emissions of a vehicle in real-time while the vehicle is in motion and subject to forces acting on the instrument that may include vibrations, an aspect change of an instrument relative to gravity (e.g. tilt), change in momentum (e.g. acceleration/deceleration), etc. A specific example of such a system may be referred to as PEMS (portable emissions measurement system) for RDE (Real-Time Driving Emissions).
Those of ordinary skill understand that “interferometer” instruments that utilize FTIR technology typically include what is referred to as a modulator that can be extremely sensitive to vibrations, tilts, and other similar conditions. For example some interferometer embodiments are particularly susceptible to externally applied forces which can result in 1) introduction of aberrations into the data, yielding poor data quality; and/or 2) upset of the control system of the interferometer, causing it to stop scanning temporarily, yielding no data for several seconds. In some cases the externally applied forces could also prevent the instrument from starting to scan from a stopped or off position.
For a continuous measurement system, such as PEMS, it is extremely desirable for the interferometer to produce substantially uninterrupted data (e.g. as few data dropouts as possible) and of substantially equal quality to that which could be obtained in a laboratory environment. One particular type of interferometer, referred to as a “Michelson interferometer” can be configured in a cost effective and compact form desirable for PEMS applications, however Michelson interferometer embodiments typically suffer from issues that result from externally applied forces as described above. For example, a Michelson interferometer works by using a Beamsplitter to reflect and transmit incoming light. One beam path of light reflects off of a stationary mirror, and the other beam path of light reflects off of a moving mirror. These two beams recombine at the Beamsplitter and exit the interferometer. As the moving mirror oscillates back and forth, interference patterns between the two light beams create a modulated signal. This modulated light signal then interacts with a sample, and is finally measured by a detector. In addition, a laser beam of a fixed, known frequency is passed through the interferometer and detected with a separate detector. This laser signal is used to sample the modulated light at a known position spacing (e.g. 1 divided by the frequency of the laser), and the laser signal is also used to control the velocity of the moving mirror through a servo motor. This measured signal in position space is Fourier transformed, resulting in a spectrum in frequency space. Additional description of Michelson interferometers are discussed in “Fourier Transform Infrared Spectroscopy” by Griffiths and de Haseth (Griffiths, Peter R., and James A. De Haseth. Fourier Transform Infrared Spectroscopy. 2nd ed. Hoboken, N.J.: John Wiley & Sons, 2007), which is hereby incorporated by reference herein in its entirety for all purposes.
Those of ordinary skill in the related art appreciate that Michelson interferometer embodiments are very precise instruments and usually incorporate one or more mechanisms to address variations that can occur through physical and/or environmental fluctuations. For example, some embodiments include a dynamic alignment system that tilts the stationary mirror to account for minor misalignments in the interferometer, as well as to account for thermal drift of the interferometer. The dynamic alignment system typically has its own control system separate from the servo motor control system that controls the moving mirror. Both the dynamic alignment and servo motor control systems use the laser signal as the measurement, but in different ways. The moving mirror servo motor control system uses what are referred to as “zero-crossings” of the laser as a measurement of the velocity of the moving mirror. The dynamic alignment system typically uses three separate laser signals separated in the plane of the beam, and uses a control system to lock the phases of the three laser signals in place to some defined phase separation. Minor misalignments in the interferometer normally cause the phase separations of these laser signals to vary throughout the oscillations of the moving mirror. This well-known technique for Michelson Interferometers is described further in standard texts, such as Fourier Transform Infrared Spectroscopy by Griffiths and de Haseth, incorporated by reference above.
However, as described above even with the mechanisms that address physical and/or environmental variations typical Michelson interferometer embodiments do not address significant effects caused by externally applied forces such as, for example, those experienced in portable applications (e.g. for vehicles that include cars, trucks, aviation, etc.) or industrial applications (e.g. power plants or remote emissions monitoring stations with large fans or other devices that cause large vibrations). Further, it is very challenging to develop a Michelson interferometer embodiment that addresses substantial effects caused by externally applied forces in a simple and cost effective manner. For example, those of ordinary skill in the art appreciate that technologies that include what may be referred to as “Quadrature” technologies exist that could address some effects caused by externally applied forces. However, the level of complexity and cost associated with such technologies is prohibitive to portable and industrial applications.
Therefore, there is a need for an improved design of an interferometer that makes it more resistant to externally applied forces in a relatively simple and inexpensive manner.